Cationic polymerization pairs

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

When concentrated sulphuric acid alone was used as the initiator, the polymerization was found to follow a different path. It is well known that Bronsted acids can function as cationic/pseudocationic initiators in the oligomerization of olifins [174]. If the counter ion has a higher nucleophilicity as it forms cation-conjugate pairs, which collapse rapidly, polymerization will not take place. As the counter ion in the case of sulphuric acid is not very strong compared to the cation, oligomerization can take place, but may not be to a very high molecular weight. This, however, depends on the nature of the... 

Cationic polymerization of tetrahydrofuran with the or bis(3,5-di-bromomethyl) benzoyl peroxide and AgPp6 pairs of 4,4 -bromomethyl benzoyl peroxide and AgPp6 yield active poly-THP having peroxide group in the main... 

Ledwith, A. and Sherrington, D. C. Stable Organic Cation Salts Ion Pair Equilibria and Use in Cationic Polymerization. Vol. 19, pp. 1 — 56. 

These various structures show characteristic differences of the reactivity during the propagation step. When one observes cationic polymerizations, the propagation via free ions takes place from 10 to 100 times faster than that via ion pairs 1-2). This ratio should be valid for anions from Lewis acids as well as those from protic acids. 

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

We propose that polymerizations in which there are two or more propagating species be termed enieidic (from Greek enioi meaning several and eidos meaning form) in order to avoid using the over-worked term polymorphic the terms monoeidic and dieidic will also be found useful. Consideration of the whole field of cationic polymerization shows that simultaneous propagation by free ions and paired ions is only one of several types of enieidic polymerization. 

Systems in which several chemically different chain carriers coexist constitute another type of enieidic polymerization. One example of this would be a polymerization in which concurrent co-catalysis by water and by alkyl halide solvent produced anions MtXBOH and MtX B+1. In that case one must assume a priori that the cations forming pairs with the two different anions will have different kinetic constants. 

In the present context the word termination is applied not to the breaking-off of a physical chain, i.e., the cessation of growth of a particular molecule, but to the complete destruction of a kinetic unit, which means the irreversible annihilation of one ion pair. This kinetic termination, which is a well-understood feature of radical polymerizations, is a comparatively rare event in cationic polymerizations it may occur in several different ways and in some systems not at all. 

Fontana et al. (1948, 1952) showed that the kinetics of the cationic polymerization of C3H6 by AlBr3 and HBr in an hydrocarbon solvent can be explained on the assumption that the alkene forms complexes with the growing cations, which might be unpaired or paired ... 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

It is generally accepted that there is little effect of counterion on reactivity of ion pairs since the ion pairs in cationic polymerization are loose ion pairs. However, there is essentially no experimental data to unequivocally prove this point. There is no study where polymerizations of a monomer using different counterions have been performed under reaction conditions in which the identities and concentrations of propagating species are well established. (Contrary to the situation in cationic polymerization, such experiments have been performed in anionic polymerization and an effect of counterion on propagation is observed see Sec. 5-3e-2.)... 

The rate of living cationic polymerization is expressed as the rate of propagation of ion pairs... 

It has previously been shown that large changes can occur in the rate of a cationic polymerization by using a different solvent and/or different counterion (Sec. 5-2f). The monomer reactivity ratios are also affected by changes in the solvent or counterion. The effects are often complex and difficult to predict since changes in solvent or counterion often result in alterations in the relative amounts of the different types of propagating centers (free ion, ion pair, covalent), each of which may be differently affected by solvent. As many systems do not show an effect as do show an effect of solvent or counterion on r values [Kennedy and Marechal, 1983]. The dramatic effect that solvents can have on monomer reactivity ratios is illustrated by the data in Table 6-10 for isobutylene-p-chlorostyrene. The aluminum bromide-initiated copolymerization shows r — 1.01, r2 = 1.02 in n-hexane but... 

Scheme 12.13 Cationic polymerization from an ion pair formed from silanol and diaryl halide on silica surface.

Thus the growing anionic chain can assume at least two identities the free anion and the anion-cation ion pair (several types of solvated ion-pairs can also be considered). Furthermore, the kinetics of these propagation reactions, which generally show a fractional dependency on chain-end concentration ranging from one-half to unity, can best be explained by assuming that the monomer can react with both the free anion and the ion-pair (4, 5, 60, but at different rates. Thus, for example, in the polymerization of styrene by organosodium, the rate of polymerization (Rp) can be expressed as... 

In order to explain the field effects observed for the cationic polymerizations, we have earlier proposed a kinetic scheme based on the two-state polymerization mechanism and on the field-facilitated dissociation hypothesis (11). Though the assumptions involved in the proposed interpretation turn out to be partly invalid in the light of the experimental data accumulated most recently (15), it is still necessary to give an outline of the scheme. We assumed that, by the initiation reaction between initiator molecules (C) and monomer molecules (M), active species of an ion-pair type (My) are produced, a portion of which dissociates into active species of a free ion type (Mf) and gegenions (C ). The propagation, monomer transfer and termination can be effected by the free ions and ion pairs. A dissociation equilibrium is established between the free ions and ion pairs, which can be characterized by a dissociation constant K. Then we have ... 

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